Vacuum plasma processors are used to deposit materials on and etch materials from workpieces that are typically semiconductor, dielectric and metal substrates. A gas is introduced into a vacuum plasma processing chamber where the workpiece is located. The gas is ignited into a plasma in response to an r.f. electric or electromagnetic field. The r.f. field is provided by a reactive impedance element, usually either an electrode array or a coil which couples both magnetic and electrostatic r.f. fields to the gas. The reactive impedance element is connected to an r.f. source having an r.f. frequency and sufficient power such that the gas is ignited into the plasma. Connections between the source and the coil are usually by way of a relatively long coaxial cable, connected directly to the r.f. source, and a resonant matching network, connected between the cable and reactive impedance element. The matching network includes a pair of variable reactances, adjusted to match the impedance of the source to the load it is driving.
The load seen by the source is subject to substantial variations. The load has a relatively high impedance prior to ignition of the gas into a plasma state. In response to the plasma being ignited, the load impedance drops substantially due to the presence of the charge carriers, i.e., electrons and ions, in the excited plasma. The ignited plasma impedance also changes substantially due to variations in the plasma flux, i.e. the product of the plasma density and the plasma charge particle velocity. Hence, matching the source to the load to provide efficient transfer of power from the source to the load is somewhat difficult.
In the past, the same technique which is used to maintain a matched condition between the source and load during normal operation of the ignited plasma has been used to control the variable reactances of the matching network at the time the gas is ignited into a plasma. This technique involves simultaneously varying both variable reactances to achieve a matched condition between the impedance seen looking into the output terminals of the source and the impedance seen by the source looking from its output terminals into the cable driving the matching network. In this technique, the values of the two reactances are simultaneously varied until (1) there is approximately a zero phase difference between the voltage and current supplied by the source to the cable and (2) the real impedance component seen looking into the source output terminals approximately equals the real impedance seen looking from the source output terminals into the cable.
It has been found that, in certain circumstances, this prior art approach is completely unsatisfactory because the gas is never ignited into a plasma. The values of the reactances are simultaneously varied in such a way that the power delivered to the reactive impedance element which produces the electric or electromagnetic field is never adequate for plasma ignition. In other situations, ignition is finally reached after a considerable length of time is spent changing the values of the reactances. The values of the reactances are simultaneously varied in a haphazard way, with no determination made as to what is the optimum direction to change the values of the reactances from initial values thereof.
It has also been suggested that the value of only one of the reactances of the matching network be changed until plasma ignition is obtained; see the co-pending, commonly assigned application Ser. No. 08/580,706, now U.S. Pat. No. 5,793,162, issued Aug. 11, 1998 of Barnes et al, entitled APPARATUS FOR CONTROLLING MATCHING NETWORK OF A VACUUM PLASMA PROCESSOR AND MEMORY FOR SAME, filed Dec. 29, 1995. It has been found, however, that this approach is not always reliable. Under certain conditions, varying only one of the reactances does not enable sufficient power to be coupled from the source to the chamber to ignite the gas into the plasma.
It is, accordingly, an object of the present invention to provide a new and improved method of and apparatus for controlling the reactances of a matching network connected between an r.f. source and a vacuum plasma processing chamber to provide sufficient power to the chamber so a gas in the chamber is reliably ignited to a plasma.
Another object of the invention is to provide a new and improved method of and apparatus for controlling reactances of a matching network connected between an r.f. source and a plasma processing chamber in such a way that the values of the reactances are varied to provide rapid ignition of a gas in the chamber into a plasma.
Another object of the invention is to provide a new and improved method of and apparatus for varying the values of reactances of a matching network connected between an r.f. source and a vacuum plasma processing chamber so the values of the reactances are varied in the correct direction to maximize a function indicative of the power coupled from an r.f. source to the chamber.